A fluid reservoir stores fluid and provides a constant
source of fluid for the system. In an automatic transmission, the reservoir is the pan, typically located at the bottom of the transmission case. ATF is forced out of the pan by atmospheric pressure and into the pump and is then returned to it after it has circulated through the selected circuits. A transmission dipstick placed within a filler tube is typically used to check
the level of the fluid and to add ATF to the transmission. Other transmissions have a side plug on the pan or the transmission to check and replenish fluid level